Re: [PATCH 1/3] rcu: Use static initializer for krc.lock

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Apr 16, 2020 at 11:05:15PM -0400, Joel Fernandes wrote:
> On Thu, Apr 16, 2020 at 11:34:44PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2020-04-16 14:00:57 [-0700], Paul E. McKenney wrote:
> > > 
> > > We might need different calling-context restrictions for the two variants
> > > of kfree_rcu().  And we might need to come up with some sort of lockdep
> > > check for "safe to use normal spinlock in -rt".
> > 
> > Oh. We do have this already, it is called CONFIG_PROVE_RAW_LOCK_NESTING.
> > This one will scream if you do
> > 	raw_spin_lock();
> > 	spin_lock();
> > 
> > Sadly, as of today, there is code triggering this which needs to be
> > addressed first (but it is one list of things to do).
> > 
> > Given the thread so far, is it okay if I repost the series with
> > migrate_disable() instead of accepting a possible migration before
> > grabbing the lock? I would prefer to avoid the extra RT case (avoiding
> > memory allocations in a possible atomic context) until we get there.
> 
> I prefer something like the following to make it possible to invoke
> kfree_rcu() from atomic context considering call_rcu() is already callable
> from such contexts. Thoughts?
> 
> (Only build tested)
> ---8<-----------------------
> 
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> Subject: [PATCH] rcu/tree: Avoid allocating in non-preemptible context for
>  PREEMPT_RT kernels
> 
> Per recent discussions, kfree_rcu() is a low-level facility which should be
> callable in atomic context (raw spinlock sections, IRQ disable sections etc).
> 
> However, it depends on page allocation which acquires sleeping locks on
> PREEMPT_RT.
> 
> In order to support all usecases, avoid the allocation of pages for
> PREEMPT_RT. The page allocation is just an optimization which does not
> break functionality. Further, in future patches the pages will be
> pre-allocated reducing the likelihood that page allocations will be
> needed.
> 
> We also convert the spinlock_t to raw_spinlock_t so that does not sleep
> in PREEMPT_RT's raw atomic critical sections.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/rcu/tree.c | 42 +++++++++++++++++++++++++-----------------
>  1 file changed, 25 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f288477ee1c26..ba831712fb307 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2905,7 +2905,7 @@ struct kfree_rcu_cpu {
>  	struct kfree_rcu_bulk_data *bhead;
>  	struct kfree_rcu_bulk_data *bcached;
>  	struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> -	spinlock_t lock;
> +	raw_spinlock_t lock;
>  	struct delayed_work monitor_work;
>  	bool monitor_todo;
>  	bool initialized;
> @@ -2939,12 +2939,12 @@ static void kfree_rcu_work(struct work_struct *work)
>  	krwp = container_of(to_rcu_work(work),
>  			    struct kfree_rcu_cpu_work, rcu_work);
>  	krcp = krwp->krcp;
> -	spin_lock_irqsave(&krcp->lock, flags);
> +	raw_spin_lock_irqsave(&krcp->lock, flags);
>  	head = krwp->head_free;
>  	krwp->head_free = NULL;
>  	bhead = krwp->bhead_free;
>  	krwp->bhead_free = NULL;
> -	spin_unlock_irqrestore(&krcp->lock, flags);
> +	raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  
>  	/* "bhead" is now private, so traverse locklessly. */
>  	for (; bhead; bhead = bnext) {
> @@ -3047,14 +3047,14 @@ static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp,
>  	krcp->monitor_todo = false;
>  	if (queue_kfree_rcu_work(krcp)) {
>  		// Success! Our job is done here.
> -		spin_unlock_irqrestore(&krcp->lock, flags);
> +		raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  		return;
>  	}
>  
>  	// Previous RCU batch still in progress, try again later.
>  	krcp->monitor_todo = true;
>  	schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
> -	spin_unlock_irqrestore(&krcp->lock, flags);
> +	raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  }
>  
>  /*
> @@ -3067,16 +3067,16 @@ static void kfree_rcu_monitor(struct work_struct *work)
>  	struct kfree_rcu_cpu *krcp = container_of(work, struct kfree_rcu_cpu,
>  						 monitor_work.work);
>  
> -	spin_lock_irqsave(&krcp->lock, flags);
> +	raw_spin_lock_irqsave(&krcp->lock, flags);
>  	if (krcp->monitor_todo)
>  		kfree_rcu_drain_unlock(krcp, flags);
>  	else
> -		spin_unlock_irqrestore(&krcp->lock, flags);
> +		raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  }
>  
>  static inline bool
>  kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
> -	struct rcu_head *head, rcu_callback_t func)
> +	struct rcu_head *head, rcu_callback_t func, bool alloc)
>  {
>  	struct kfree_rcu_bulk_data *bnode;
>  
> @@ -3092,6 +3092,10 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
>  		if (!bnode) {
>  			WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
>  
> +			/* If allocation is not allowed, don't do it. */
> +			if (!alloc)
> +				return false;
> +
>  			bnode = (struct kfree_rcu_bulk_data *)
>  				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
>  		}
> @@ -3138,11 +3142,15 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  {
>  	unsigned long flags;
>  	struct kfree_rcu_cpu *krcp;
> +	bool alloc = true;
> +
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> +		alloc = false;
>  
>  	local_irq_save(flags);	// For safely calling this_cpu_ptr().
>  	krcp = this_cpu_ptr(&krc);
>  	if (krcp->initialized)
> -		spin_lock(&krcp->lock);
> +		raw_spin_lock(&krcp->lock);
>  
>  	// Queue the object but don't yet schedule the batch.
>  	if (debug_rcu_head_queue(head)) {
> @@ -3156,7 +3164,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  	 * Under high memory pressure GFP_NOWAIT can fail,
>  	 * in that case the emergency path is maintained.
>  	 */
> -	if (unlikely(!kfree_call_rcu_add_ptr_to_bulk(krcp, head, func))) {
> +	if (unlikely(!kfree_call_rcu_add_ptr_to_bulk(krcp, head, func, alloc))) {
>  		head->func = func;
>  		head->next = krcp->head;
>  		krcp->head = head;
> @@ -3173,7 +3181,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  
>  unlock_return:
>  	if (krcp->initialized)
> -		spin_unlock(&krcp->lock);
> +		raw_spin_unlock(&krcp->lock);
>  	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(kfree_call_rcu);
> @@ -3205,11 +3213,11 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
>  
>  		count = krcp->count;
> -		spin_lock_irqsave(&krcp->lock, flags);
> +		raw_spin_lock_irqsave(&krcp->lock, flags);
>  		if (krcp->monitor_todo)
>  			kfree_rcu_drain_unlock(krcp, flags);
>  		else
> -			spin_unlock_irqrestore(&krcp->lock, flags);
> +			raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  
>  		sc->nr_to_scan -= count;
>  		freed += count;
> @@ -3236,15 +3244,15 @@ void __init kfree_rcu_scheduler_running(void)
>  	for_each_online_cpu(cpu) {
>  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
>  
> -		spin_lock_irqsave(&krcp->lock, flags);
> +		raw_spin_lock_irqsave(&krcp->lock, flags);
>  		if (!krcp->head || krcp->monitor_todo) {
> -			spin_unlock_irqrestore(&krcp->lock, flags);
> +			raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  			continue;
>  		}
>  		krcp->monitor_todo = true;
>  		schedule_delayed_work_on(cpu, &krcp->monitor_work,
>  					 KFREE_DRAIN_JIFFIES);
> -		spin_unlock_irqrestore(&krcp->lock, flags);
> +		raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  	}
>  }
>  
> @@ -4140,7 +4148,7 @@ static void __init kfree_rcu_batch_init(void)
>  	for_each_possible_cpu(cpu) {
>  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
>  
> -		spin_lock_init(&krcp->lock);
> +		raw_spin_lock_init(&krcp->lock);
>  		for (i = 0; i < KFREE_N_BATCHES; i++) {
>  			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
>  			krcp->krw_arr[i].krcp = krcp;

I have the same view on it how to handle it in PREEMPT_RT. Basically if
we can store the pointer in array we do it, if a current context is not
preemptible just build the list using rcu_head and queuing it for
farther process. If context is preemptible we will utilize our "array
pointer" approach, so the performance optimization will be in place
on CONFIG_PREEMPT_RT kernel.

I think this is an easiest way of making it PREEMPT_RT friendly. Also
we need to add static initialize of "raw spin lock". Split this patch
to:
    - converting to raw spinlocks;
    - make it statically initialized;
    - bypass the page allocator if RT and not preemptable.

--
Vlad Rezki